Method for regulating an electrolytic cell for aluminum production

ABSTRACT

The invention concerns a method for regulating an electrolytic cell designed for aluminium production by fused bath electrolysis enabling accurate and reliable control, in time and optionally in space, of the amount of alumina dissolved in the electrolytic solution ( 13 ). The inventive regulating method is characterized in that it comprises: a) controlling the addition of a specific amount Qo of alumina in the solution ( 13 ); b) determining the value of an indicator A of the added amount Q of alumina which is rapidly dissolved in the solution ( 13 ); c) determining the amount ΔQ of added alumina which is not rapidly dissolved in the solution ( 13 ); d) adjusting at least one setting means and/or one control operation and/or at least an intervention on the cell, on the basis of the value obtained for the amount ΔQ, so as to maintain it or bring it down to a value lower than a reference value S. In an advantageous embodiment of the invention, the indicator A is determined by analyzing at least one electrical measurement on the cell.

FIELD OF THE INVENTION

[0001] The invention relates to a method for regulating an aluminium production cell by electrolysis of alumina dissolved in an electrolyte based on molten cryolite, particularly according to the Hall-Héroult process.

STATE OF THE ART

[0002] Aluminium metal is produced industrially by fused bath electrolysis, namely by electrolysis of alumina in solution in a bath based on molten cryolite called an electrolyte bath, particularly using the well-known Hall-Heroult process.

[0003] The electrolyte bath is contained in pots called “electrolytic pots”, comprising a steel shell that is lined on the inside with refractory and/or insulating materials, and a cathode assembly located in the bottom of the pot. Anodes are partially immersed in the electrolyte bath. The anodes are typically made of a carbonaceous material, although they may be partially or wholly composed of an “inert” material such as a metallic material or a ceramic/metal (or “cermet”) composite. The expression “electrolytic cell” normally denotes the assembly comprising an electrolytic pot and one or more anodes.

[0004] The electrolytic current that circulates in the electrolyte bath and the pad of liquid aluminium through anodes and cathode elements, causes alumina reduction reactions and also provides a means of maintaining the electrolyte bath at a temperature of the order of 950° C. due to the Joule effect. The electrolytic cell is fed with alumina regularly so as to compensate for alumina consumption caused by the electrolysis reactions.

[0005] The productivity and the current efficiency of an electrolytic cell are influenced by several factors, such as the intensity and distribution of the electrolytic current, the bath temperature, the content of dissolved alumina and the acidity of the electrolyte bath, etc., that interact with each other. For example, the melting temperature of a cryolite based bath decreases with the excess of aluminium trifluoride (AlF₃) compared with the nominal composition (3 NaF.AlF₃). The melting temperature is also influenced by the presence of compounds such as CaF₂, MgF₂ or LiF. In modem aluminium reduction plants, operating parameters are adjusted to aim at Current efficiencies exceeding 90%. Therefore, running an electrolytic cell requires precise control over its operating parameters such as its temperature, alumina content, acidity, etc., so as to keep them at determined set values. Several regulating methods have been developed to achieve this objective. These methods normally apply either to regulation of the alumina content of the electrolyte bath, or regulation of its temperature, or regulation of its acidity, in other words excess AlF₃.

[0006] One of the essential factors for assuring uniform operation of an aluminium production pot by electrolysis of alumina dissolved in a molten electrolytic bath based on cryolite is to maintain an appropriate content of alumina dissolved in this electrolyte and consequently to adapt quantities of alumina added into the bath to match alumina consumption in the pot.

[0007] Excess alumina creates a risk that slag accumulation could occur on the bottom of the pot due to deposits of undissolved alumina that could be transformed into hard plates that could electrically isolate part of the cathode.

[0008] This phenomenon then encourages the formation of very high horizontal electrical currents in the metal of the pots that interact with magnetic fields to stir the metal pad and cause instability at the bath-metal interface.

[0009] Conversely, a shortage of alumina can in particular cause the appearance of the “anode effect”, in other words polarization of an anode with sudden increase in the voltage at the cell terminals and release of large quantities of fluorinated and fluoro-carbonaceous (CF_(x)) products, whose high capacity for absorption of infrared rays promotes the greenhouse effect.

[0010] Therefore, the need to maintain the content of alumina dissolved in the electrolyte within precise and relatively narrow limits, and therefore to adapt added alumina to the needs of the cell, has encouraged those skilled in the art to develop automatic methods for feeding and regulating alumina in electrolytic pots. This need has become an obligation with the use of “acid” electrolytic baths (with a high content of AlF₃), to lower the operating temperature of the pot by 10 to 15° C. (about 950° C. instead of the normal value of 965° C.), or even below 950° C. due to the addition of compounds such as CaF₂, MgF₂ or LiF in order to thus achieve Current efficiencies of at least 94%. It is then essential to be able to adjust the alumina content within a very precise and very narrow concentration range (typically from 1% to 3.5%, and preferably between 1.5 and 2.5%, in cells comprising anodes made of a carbonaceous material), considering the reduction in the solubility rate of alumina related to the new composition and the drop in the bath temperature.

[0011] In industrial processes, it is known that an indirect evaluation of alumina contents can be made by monitoring an electrical parameter representative of the alumina concentration in the said electrolyte. This parameter is usually the variation of the resistance R at the terminals of the pot powered at a voltage U, including a counter-electromotive force E evaluated for example at 1.65 volts and through which a current I passes such that R=(U−E)/I. Typically, methods for regulating alumina content consist of modulating the alumina feed as a function of the value of R and its variation with time. This basic principle has been defined in many patents until very recently (for example see French patent application FR 2 749 858 corresponding to American patent U.S. Pat. No. 6,033,550). These regulating methods do not directly take account of the rate of dissolution of alumina added in the electrolyte.

[0012] This regulation mode provides a means of maintaining the alumina content of the bath within a narrow and low range and thus to obtain Current efficiencies of the order of 95% with acid baths, simultaneously and significantly reducing the amount (or frequency) of anode effects on pots that are counted as the number of anode effects per pot and per day (AE/pot/day), under the term “anode effect rate”.

[0013] Recent developments in technologies for production of aluminium by fused bath electrolysis have, however, introduced additional constraints in precise control of the amount of alumina dissolved in the electrolyte. Firstly, the increase in the nominal intensity of electrolytic pots using anodes made of a carbonaceous material (by modifying existing pots or by developing new generation pots capable of reaching or exceeding 500 kA) has been accompanied by a large relative reduction in the bath volume and a large relative increase in the number of alumina doses introduced into the pot. These changes have exacerbated phenomena that could quickly vary the amount of alumina dissolved in the bath. Secondly, pots equipped with so-called inert anodes, for which the bath can be at a temperature of below 950° C. and/or for which the concentration of dissolved alumina can be greater than 3%, or even more, are also sensitive to precipitation of alumina. In this case, regulation techniques based on a simple resistivity measurement are difficult to apply since the resistivity then varies only slightly with the concentration of alumina.

[0014] These different developments have increased the importance of precise and reliable control over the amount of alumina dissolved in the bath. Therefore the applicant has attempted to find economic solutions to these difficulties, that can be applied industrially.

DESCRIPTION OF THE INVENTION

[0015] An object of the invention is a method for regulating an electrolytic cell for aluminium production by fused bath electrolysis, in other words by passing a current in an electrolyte bath based on molten cryolite and containing dissolved alumina, particularly according to the Hall-Héroult process.

[0016] The regulating method according to the invention includes the addition of alumina into the electrolyte bath of an electrolytic cell, and is characterized in that it comprises:

[0017] a) controlling the addition of a specific amount Qo of alumina in the bath,

[0018] b) determining the value of an indicator A of the amount Q of alumina which is rapidly dissolved in the bath,

[0019] c) determining the amount AQ of alumina which is not rapidly dissolved in the bath,

[0020] d) adjusting at least one adjustment means and/or at least one control operation, and/or at least one intervention on the cell, as a function of the value obtained for the amount AQ so as to maintain it or bring it down to a value lower than a reference value S.

[0021] Alumina is typically added in the form of doses of a known amount Qo, output by an automatic device or proportioner at a period P.

[0022] The indicator A is advantageously determined from an electrical measurement on the electrolytic cell capable of detecting variations of the electrical characteristics of the bath caused by the fraction of added alumina put into solution in the bath.

[0023] The applicant has noted that, surprisingly, additions of alumina created a change in the voltage over a time scale of the order of a few seconds, ascribed to the dissolution kinematics. The applicant also realised the importance of taking account of alumina dissolution rates, and particularly fast and slow dissolution rates.

[0024] The fast dissolution rate typically corresponds to alumina grains that explode as they enter the bath (under the effect of high temperature and evaporation of water chemically bonded to alumina), dispersing fine particles of alumina that immediately pass into suspension in the bath and consequently dissolve in it quickly, typically within a few seconds, in other words in a time less than the time P marking the arrival of the next dose.

[0025] The slow dissolution rate typically corresponds to alumina grains that lump together when they penetrate into the bath, solidifying the surrounding bath, forming a solid mass denser than the bath and aluminium that is deposited and accumulates on the bottom of the crucible. Alumina trapped in this way can then only dissolve in the bath very slowly and very gradually over a long period of time, typically several hours or even several days.

[0026] Indicator A may possibly be partly or wholly determined by sampling or using probes, typically chemical, physical or physicochemical probes such as optical probes (Raman or other).

[0027] The amount Q of alumina that dissolves quickly in the bath can be determined by calibrating the said indicator A, typically by modelling and/or making statistical measurements on cells of the same type operating under similar conditions. The amount AQ of alumina that does not dissolve in the bath quickly is typically determined by subtracting the quantities Q_(o) and Q, in other words ΔQ=Q_(o)−Q. This calculation method can be used if alumina is injected in doses of known quantities Q_(o). In some cases, the amount ΔQ may be determined by more sophisticated methods, such as a digital integral calculation, for example taking account of temperature effects introduced by additions of alumina, the position of measurement or sampling points, and other factors influencing this magnitude.

[0028] The said modifications to at least one adjustment means of the cell and/or at least one control operation can advantageously be combined.

FIGURES

[0029]FIG. 1 shows a cross-section through a typical electrolytic cell using prebaked anodes made of a carbonaceous material.

[0030]FIG. 2 illustrates a method of measuring the voltage on an electrolytic pot.

[0031]FIG. 3 illustrates a method of determining the specific electrical resistance of the electrolytic cell.

[0032]FIG. 4 shows the voltage signals measured on an electrolytic cell.

DETAILED DESCRIPTION OF THE INVENTION

[0033] As illustrated in FIG. 1, an electrolytic cell (1) for the production of aluminium by the Hall-Héroult electrolysis process typically comprises a pot (20), anodes (7) supported by attachment means (8,9) to an anode frame (10) and alumina feed means (11). The support and attachment means typically comprise at least one stem (9). The pot (20) comprises a steel shell (2), internal lining elements (4,4 a) and cathode elements (5,6). The internal lining elements (4,4 a) are usually blocks made of refractory materials, which may be thermal insulation. The cathode elements (5,6) comprise connection bars (or cathode bars) (6) to which electrical conductors used to transfer the electrolytic current are attached.

[0034] The lining elements (4,4 a) and the cathode elements (5,6) form a crucible inside the pot capable of containing the electrolyte bath (13) and a pad of liquid metal (12) when the cell is in operation, during which the anodes (7) are partially immersed in the electrolyte bath (13). The electrolyte bath contains dissolved alumina, and a cover (or crust) of alumina (14) usually covers the electrolyte bath. In some operating modes, the internal side walls (3) may be covered by a solidified bath layer (15).

[0035] The electrolytic current passes in the electrolyte bath (13) by means of the anode frame (10), support and attachment means (8,9), anodes (7) and cathode elements (5, 6). The cathode elements usually comprise at least one metallic cathode bar (6) (typically made of steel for traditional pots).

[0036] The purpose of the alumina feed to the cell is to compensate for the substantially continuous consumption of the cell that is essentially due to the reduction of alumina into aluminium metal. The alumina feed, that is made by adding alumina into the liquid bath (13) is usually regulated independently. The feeding means (11) typically include crustbreaker—proportioners (19) capable of penetrating through the alumina crust (14), and adding a dose of alumina in the opening (19 a) formed by digging through the alumina crust.

[0037] The aluminium metal that is produced during electrolysis normally accumulates at the bottom of the pot and a fairly sharply defined interface is formed between the liquid metal (12) and the bath based on molten cryolite (13). The position of this bath—metal interface varies with time: it rises as the liquid metal accumulates at the bottom of the pot and it drops when liquid metal is extracted from the pot.

[0038] Several electrolytic cells are usually arranged in line, in buildings called electrolytic halls, and are electrically connected in series using connecting conductors. More precisely, cathode bars (6) of a so-called “upstream” pot, are electrically connected to the anodes (7) of a so-called “downstream” pot, typically through connecting conductors (16, 17, 18) and support means (8, 9, 10) of anode (7). The cells are typically arranged so as to form two or more parallel rows. The electrolytic current thus passes in cascade from one cell to the next.

[0039] Most of these elements, or at least their functions, can be located on electrolytic pots using non-consumable anodes called “inert” anodes. In production, these anodes will release oxygen instead of carbon dioxide that is normally released by anodes made of a carbonaceous material.

[0040] According to the invention, the method for regulating an electrolytic cell (1) for production of aluminium by electrolytic reduction of alumina dissolved in an electrolyte bath (13) based on cryolite, the said cell (1) comprising a pot (20), at least one anode (7), at least one cathode element (5, 6), the said pot (20) comprising internal side walls (3) and being capable of containing a bath of liquid electrolyte (13), the said cell (1) also comprising at least one means of adjustment of the said cell (typically a mobile anode frame (10) to which the said at least one anode (7) is fixed), the said cell (1) being capable of carrying a so-called electrolytic current in the said bath, the said current having an intensity I, the aluminium produced by the said reduction possibly forming a “liquid metal pad” (12) on the cathode element(s) (5, 6), comprises the addition of alumina in the said bath and is characterised in that it comprises:

[0041] a) controlling the addition of a specific amount Q_(o) of alumina in the bath (13);

[0042] b) determining the value of an indicator A of the added amount Q of alumina which is rapidly dissolved in the bath (13);

[0043] c) determining the amount AQ of added alumina which is not rapidly dissolved in the bath (13);

[0044] d) adjusting at least one adjustment means and/or at least one control operation, and/or at least one intervention on the cell, as a function of the value obtained for the amount ΔQ so as to maintain it or bring it down to a value lower than a reference value S.

[0045] The amount Q of alumina that dissolves quickly in the bath (fast dissolution rate) corresponds to the fraction of alumina that dissolves within a time typically of the order of a few seconds to a few tens of seconds. The amount of alumina ΔQ that does not dissolve quickly in the bath (slow dissolution rate) corresponds to the fraction of alumina that is dissolved within a time typically of the order of several hours to several days. The regulating method according to the invention can be simplified by fixing a single time threshold T between the fast and the slow rates. According to this simplified variant, the amount Q corresponds to the fraction of alumina that dissolves within a time less than or equal to a determined time threshold T and the amount ΔQ corresponds to the fraction of alumina that dissolves within a time greater than the threshold T. The time threshold T is typically between 100 and 1000 seconds.

[0046] The amount ΔQ can be used as an indicator of the dissolution efficiency of added alumina. This amount can be expressed as a relative value, for example ΔQ/Q_(o) or (Q_(o)−ΔQ)/Q_(o)=Q/Q_(o).

[0047] The amount Q_(o) corresponds to a feed rate that may be continuous or discontinuous. It is typically expressed in the form of doses per unit time, typically doses of the order of 1 kg.

[0048] According to one advantageous embodiment of the invention, the said indicator A is determined starting from at least one electrical measurement on the electrolytic cell (1) capable of detecting variations of the electric characteristics of the bath (13) caused by the fraction of added alumina that is dissolved in the bath.

[0049] In particular, the indicator A may be determined from an analysis of a voltage U and/or an intensity I measured on the cell (1), possibly expressed in the form of a resistance R.

[0050] The voltage U is advantageously measured between a collector (17) and a riser (16), preferably in the bottom part (16 a) of the said riser (as illustrated in FIG. 2), which in particular reduces wiring (22) and facilitates access to voltage measurement points (24, 25).

[0051] The voltage U and/or the current I, or possibly the resistance R, may be analysed by signal processing. Known methods of signal processing can be used for this analysis, such as the spectral analysis or time analysis (for example by decomposition into wavelets or packets of wavelets, by time—frequency analysis or by synchronous analysis of several signals (possibly measured at different locations in the cell)). The signal may be processed in relation with information known elsewhere, such as add alumina orders, in order to establish correlations and possibly to determine transfer functions. These data may also be processed statistically. For example, the shape of voltage signals marked by steps originating from additions of alumina doses can be analysed by signal processing, and their number can be analysed by statistical processing.

[0052] The indicator A can also be determined from an active electrical measurement, such as a measurement of the resistivity of the bath (13) that (under some conditions) may be made by moving anodes (7) with respect to the cathode elements (5, 6). For example, the indicator A may be given by a variation of a specific resistance ARS that can be determined by a measurement process comprising:

[0053] determination of at least one first value I1 for the said intensity I, and at least one first value U1 for the voltage drop U at the terminals of the said cell (1);

[0054] calculation of a first resistance R1 starting from at least the said values I1 and U1;

[0055] displacement of the anode frame (10) by a determined distance ΔH from a so-called initial position, either upwards (when ΔH is positive) or downwards (when ΔH is negative);

[0056] determination of at least one second value 12 for the said intensity I and at least one second value U2 for the voltage drop U at the terminals of the said cell (1);

[0057] calculation of a second resistance R2 from at least the said values I2 and U2;

[0058] calculation of a resistance variation ΔR using the formula ΔR=R2−R1;

[0059] calculation of the said variation of the specific resistance ΔRS using the formula ΔRS=ΔR/ΔH.

[0060] Preferably, the measuring method also comprises (at least after determination of the values of I1, I2, U1 and U2), displacement of the anode frame (10) so as to bring it back into its initial position and to find the initial setting of the cell. The said first and second resistances (R1 and R2) can be calculated using the formula R=(U−U_(o))/I, where U₀ is a constant value typically between 1.6 and 2.0 V. For example, R1 and R2 can be given by R1=(U1−U_(o))/I1 and R2=(U2−U_(o))/I2. According to one variant of the invention, R1 and R2 can be given by an average value obtained from a determined number of values of the voltage U and the intensity I.

[0061] In practice, it has been found easier to give an order to displace the anode frame (10) for a determined time and to measure the resulting displacement AH of the frame.

[0062] As shown in FIG. 3, the resistance R is typically measured using means (23) of measuring the intensity I of the current circulating in the cell and means (21, 22) of measuring the resulting voltage drop U at the terminals of the cell (typically the resulting voltage drop between the anode frame and the cathode elements of the cell). The said resistance R is generally calculated using the equation: R=(U−U_(o))/I, where U_(o) is a constant value. These measurements of the specific resistances ΔRS may be made at regular intervals (for example every 20 minutes) and analysed statistically.

[0063] Typically, the means (21) is a voltmeter, the means (22) is an electrical conductor such as a cable or an electrical wire, and the means (23) is an ammeter.

[0064] Determination of the said indicator A by electrical measurements has the advantage of being cost-effective and that it can be automated.

[0065] The applicant also had the idea that the local nature of dissolution of alumina could be demonstrated by separate electrical measurements on the cell, in other words electrical measurements at at least two different locations of the cell.

[0066] Typically, voltage measurements could be made between different risers (16) and different cathode bars (6), advantageously close to alumina feed points (19 a) (for example close to crustbreakers—proportioners when this method of adding alumina is used).

[0067] The said adjustment, control operation and intervention on the cell may be short, medium and long term actions. The said adjustment using at least one means of adjusting the cell typically comprises at least a modification to the amount Q_(o), in other words the alumina feed rate to the cell. For example, the amount Q_(o) can be adjusted by modifying the feed frequency (in other words by modifying the number of alumina doses per unit time) and/or by modifying the added dose (in other words the alumina amount contained in each dose). These adjustments generally have a short term effect.

[0068] The said adjustment may also include at least one modification to the position of the anodes (7), for example by modifying the position of a mobile anode frame (10) either upwards or downwards, so as to modify the distance between the anodes (7) and the cathode elements (5, 6), and more precisely the anode/metal distance (AMD) when the liquid metal forms a pad (12) under the anodes. This adjustment, which is basically thermal, generally has a medium term effect.

[0069] The said at least one control operation includes, for example, the addition of a determined amount of AlF₃ into the said electrolyte bath (13). This operation generally has a long term effect.

[0070] The at least one intervention may include fast displacement of the anodes (7) to modify the interface conditions between the anodes and the bath and/or to eliminate gas bubbles that may be present under the surface of the anodes. The reference value S is normally a very small value, so that AQ is made to tend towards zero.

[0071] The anode(s) (7) may be anodes made of a carbonaceous material or non-consumable anodes. Non-consumable anodes can include a metallic material, a coated material or a cermet (in other words a ceramic—metal composite).

[0072] Test

[0073] A test was carried out on a prototype pot fitted with anodes made of a carbonaceous material with an intensity of the order of 480 kA. This pot was fitted with crustbreakers—proportioners capable of penetrating the alumina crust and injecting a determined dose of 1 kg of alumina into the opening formed by digging.

[0074] Voltage measurement points (24, 25) between some anodes and some cathode bars were fixed on this pot, as illustrated diagrammatically in FIG. 2.

[0075] The voltage was recorded during electrolysis for a period of about 1 month. This voltage had fluctuations in time of the order of 10 to 20 mV. An analysis of this signal by digital processing demonstrated voltage steps of the order of a few mV to a few tens of mV that can be attributed to additions of alumina doses by the crustbreakers—proportioners (see FIG. 4 that gives an example of a voltage U measured as a function of time t). The amplitude and shape of these steps can also be at least partly attributed to the dissolution kinematics of the alumina. The applicant had the idea of using these steps as indicators of the amount of dissolved alumina in the bath.

List of references

[0076] (1) electrolytic cell

[0077] (2) pot shell

[0078] (3) internal side wall

[0079] (4) (4 a) internal lining elements

[0080] (5) cathode element

[0081] (6) connection bar or cathode bar

[0082] (7) anode

[0083] (8) anode support means

[0084] (9) anode support and attachment means, called stem

[0085] (10) anode frame

[0086] (11) alumina feeding means

[0087] (12) liquid metal pad

[0088] (13) electrolyte bath

[0089] (14) alumina cover (or crust)

[0090] (15) solidified bath layer

[0091] (16) connecting conductor (riser)

[0092] (16 a) bottom part of a riser

[0093] (17) connecting conductor (collector)

[0094] (18) connecting conductor

[0095] (19) crustbreaker—proportioner

[0096] (19 a) alumina feed point

[0097] (20) pot

[0098] (21, 22) voltage measurement means

[0099] (23) intensity measurement means

[0100] (24) (25) electrical voltage measurement points. 

1. Method for regulating an electrolytic cell for production of aluminum by electrolytic reduction of alumina dissolved in an electrolyte bath based on cryolite, the cell comprising a pot, at least one anode, at least one cathode element, the pot comprising internal side walls and being capable of containing a bath of liquid electrolyte, the cell also comprising at least one means of adjustment of the cell, the cell being capable of carrying an electrolytic current in the bath, the current having an intensity I, the method comprising adding alumina to the bath and further comprising: a) controlling an addition of a specific amount Q_(o) of alumina in the bath; b) determining a value of an indicator A of an added amount Q of alumina which is rapidly dissolved in the bath; c) determining an amount ΔQ of added alumina which is not rapidly dissolved in the bath; d) adjusting at least one adjustment means and/or at least one control operation, and/or at least one intervention on the cell, as a function of the value obtained for the amount ΔQ, so as to maintain said value or bring said value to a second value lower than a reference value S.
 2. Method according to claim 1, wherein the amount Q corresponds to a fraction of alumina that dissolves within a time less than or equal to a determined time threshold T, and the amount ΔQ corresponds to a fraction of alumina that dissolves within a time greater than the threshold T.
 3. Method according to claim 2, wherein the time threshold T is between 100 and 1000 seconds.
 4. Method according to of claim 1, wherein the indicator A is determined starting from at least one electrical measurement on the cell capable of detecting variations of electric characteristics of the bath caused by the fraction of added alumina that is dissolved in the bath.
 5. Method according to claim 4, wherein the indicator A is determined from an analysis of a voltage U and/or an intensity I measured on the cell.
 6. Method according to claim 5, wherein the method includes signal processing.
 7. Method according to claim 4, wherein the indicator A is determined from a measurement of the resistivity of the bath.
 8. Method according to claim 4, wherein, in order to determine a local nature of dissolution of alumina, the at least one electrical measurement is performed at at least two different locations of the cell.
 9. Method according to any one of claim 1, wherein the indicator A is partly or wholly determined by sampling or using probes.
 10. Method according to any one of claim 1, wherein the amount Q is determined by calibrating the indicator A.
 11. Method according to claim 10, wherein the calibrating is done by modelling and/or making statistical measurements on at least one cell operating under similar conditions as the cell of said method.
 12. Method according to claim 1, wherein the amount ΔQ of alumina that does not dissolve in the bath is determined by subtracting quantities Q_(o) and Q, such that ΔQ=Q_(o)−Q.
 13. Method according to claim 1, wherein the adjusting comprises at least a modification to the amount Q_(o) of alumina added per unit time.
 14. Method according to claim 1, wherein the adjusting includes at least one modification to the position of the at least one anode, so as to modify a distance between the at least one anode and the cathode element.
 15. Method according to claim 1, wherein the at least one control operation includes adding a determined amount of AlF₃ into the said electrolyte bath.
 16. Method according to any one of claim 1, wherein the at least one intervention includes fast displacement of the at least one anode to modify interface conditions between the at least one anodes and the bath, and/or to eliminate gas bubbles present under the surface of the at least one anode.
 17. Method according to any one of claim 1, wherein the at least one anode is selected from the group consisting of anodes made of a carbonaceous material and non-consumable anodes.
 18. Method according to claim 17, wherein the non-consumable anodes comprise metallic anodes, coated anodes and/or anodes of a ceramic—metal composite. 